This invention relates to a device and method for implementing an antifuse device with a bipolar junction transistor (BJT).
In order to realize long term storage of digital information it is desirable to have a device that can be programmed into one of two states in a permanent, non-volatile manner. The two states of the device should be easily discernable. Such a device can readily represent one bit of information. Typically, permanent programming requires that a device retain the programmed value for an extended amount of time once programmed. For a device to be non-volatile, the device is required to retain the programmed value even after a power source is removed from the device.
A number of different types of devices are capable of implementing applications requiring permanent, non-volatile programming. One such device is an antifuse. A typical antifuse device as known in the prior art is a two terminal device. The antifuse's initial unprogrammed state is high impedance (a working approximation of an “open” circuit) while the antifuse's programmed state is low impedance (a working approximation of a “short” circuit). This is in contrast to the well-known fuse where the initial unprogrammed state is low impedance and the programmed state is high impedance. This contrast in behavior is the rationale behind the name “antifuse”.
As stated above, the antifuse is a non-volatile memory element. One particular application of non-volatile memory elements is in read-only memory (ROM). Many applications make use of ROM because of its ability to retain programmed data patterns after power has been removed, in contrast with most random access memory (RAM) systems, which require power to retain programmed data patterns. ROM is therefore known as non-volatile memory. A number of different classes of non-volatile memory are available. Some are programmed by the manufacturer and cannot be changed, others allow a user to program the device only once, and still others allow the memory to be programmed a number of times by a user. Non-volatile memory devices that can only be programmed once are known as one time programmable (OTP) devices. An example of an OTP device is a programmable ROM (PROM) device which allows a user to program the memory one time only. Antifuse devices are therefore well suited for use in OTP devices such as PROMs.
Antifuses are also used in conjunction with other devices to effectively remove offsets from certain outputs of electrical circuits, a practice known as “trimming”. Those skilled in the art of circuit design are be familiar with the concept of trimming, which involves fine tuning a particular parameter(s) of a device so as to compensate for random variation of that parameter. Adjusting a voltage reference's output voltage to a precise value is a typical example of trimming.
An antifuse device is a normally high resistance element that is programmed to a low resistance by applying a sufficiently large voltage, known as the programming voltage, such that the impedance between the first terminal and the second terminal is substantially reduced, resulting in a permanent low resistance path between the two terminals. This change in resistance cannot be reversed, which makes antifuses useful in applications in which the programmed data pattern will be permanent, i.e., non-volatile.
The programming voltage of a typical antifuse as known in the prior art requires a sufficiently high voltage level in order to program the antifuse. Typically, the programming voltage is much higher than the logic level voltages used in most circuits. A typical programming voltage required to program an antifuse device known in the prior art is greater than 10 volts. The high voltage required by the antifuse device requires a circuit designer to make modifications to the other circuits on an integrated circuit, to ensure they can withstand voltages of greater than 10 volts without being damaged. High voltage circuit design often requires special high voltage devices and additional resources to implement. Thus, it is desirable to create an antifuse which does not require high voltage levels to program.